1. Field of the Invention
This invention relates to rotary offset lithographic duplicating machines and more particularly to auxiliary form roller apparatus therefor which may be easily installed on virtually all of the commercially-available types of such machines to provide more uniform ink distribution on the master cylinder of the machines.
2. Description of the Prior Art
In the rotary offset lithographic duplicating process, a rotating master cylinder is provided with a detachable master sheet or "master" upon which the image to be reproduced is fixed. The image area is treated with an ink-attractive substance which will accept ink but repel a moisturizing or dampening liquid. The master cylinder is supplied with ink by one or more machine form rollers which are coupled to an ink fountain by means of an ink transfer roller train so that the image on the master is coated with ink from the fountain. A moisture roller is arranged to engage the master cylinder so that the non-image areas of the master are coated with moisture or dampening agent. The dampening agent prevents the ink from the form rollers from adhering to the clear, non-image bearing area of the master. As the master cylinder rotates, it engages a rotatable blanket cylinder which receives the ink from the image area of the master and transfers it to paper which is fed between the blanket cylinder and an impression cylinder which engages the blanket cylinder. In this manner, the image is transferred to the paper.
A principal problem associated with this type of reproduction process is that the ink from the machine form rollers is not always uniformly distributed over the image areas of the master. When this happens, some of the image areas may be lighter in tone or color than the other image areas or a fairly large image area intended to have a solid, uniform color may have a washed-out or non-uniform appearance. One solution to this problem is, of course, to provide the master cylinder with more machine form rollers so that the ink from the ink fountain is distributed more uniformly over the surface of the master cylinder and is milled into finer films for clean half-tones and bold solids. Although some newer models of offset duplicating machines have been provided with an additional form roller for this purpose, the vast majority of older duplicating machines in use at the present time have only two form rollers.
At first glance, it would appear to be a simple problem to merely add an additional form roller to an older duplicating machine to improve ink distribution. However, the common design features of the many different types of older duplicating machines make it extremely difficult to install an additional or auxiliary form roller to the machines after the machines have been built. The average owner of such older types of duplicating machines does not have the time or the technical expertise to modify his existing duplicating machines to add an additional or auxiliary form roller. Furthermore, the basic designs of the older duplicating machines are so many and varied as to heretofore prevent the development of suitable auxiliary form roller apparatus which may be readily installed in virtually all of the older types of machines by persons having only average mechanical skills.